As the density of semiconductor devices increases and as the size of circuit elements becomes smaller, resistance-capacitance (RC) delay time increasingly affects integrated circuit performance. Low-k dielectrics, therefore, are used to reduce the RC delay. Low-k dielectrics are particularly useful as inter-layer dielectrics and inter-metal dielectrics. However, low-k dielectrics present problems during processing, especially during the fabrication of the conductive material used to make interconnects.
After formation, low-k dielectric materials typically go through various post-formation processes, such as etching, ashing, wet cleaning, chemical mechanical polishing, etc., and the properties of the low-k dielectric materials are adversely affected. For example, due to the chemicals used, contamination may occur. When there is an open pore in the dielectric material, processing fluids can enter the pores, thereby causing corrosion, mechanical damage, or an increase in the dielectric constant. Additionally, materials involved in the etching and ashing processes, such as fluorine and nitrogen, enter the porous dielectric films and further damage low-k dielectric materials. Pore damage may also cause a surface that is preferably hydrophobic to become hydrophilic.
Chemicals used in the post-formation processes cause bonds between the low-k dielectric elements to break, thus degrading the mechanical strength of the low-k dielectric materials. Missing bonds also destabilize the low-k dielectric materials, since undesired elements may form bonds with the low-k dielectric materials, causing a decrease in mechanical strength and an increase in the k value. It has been found that a low-k dielectric film having a k value of about 2.56 and a hardness of about 1.4 GPa after its formation will have a k value of about 3.02 and a hardness of about 1.1 GPa after etching, ashing, and chemical mechanical polishing.
Due to the previously discussed reasons, after the post-formation processes, the low-k dielectric materials need to be restored. However, baking processes, which are typically used in existing IC manufacturing, can only remove moisture trapped in the low-k dielectric films, but can neither repair damage nor remove fluorine and nitrogen from the low-k dielectric films.
Therefore, there is the need for a method to restore the low-k dielectric films by removing contaminators and repairing bonds.